1. Field of the Invention
This invention relates generally to analog circuitry and more particularly to operational amplifiers.
2. Background of the Invention
Operational amplifiers are known to be used in a wide variety of applications. For instance, operational amplifiers may be used as buffers, amplifiers, power amplifier drivers, etc., and are used in such forms in an almost endless list of electronic devices. For example, operational amplifiers are readily used in radio devices, televisions, telephones, wireless communication devices, entertainment equipment, etc.
When an operational amplifier is employed as a power amplifier driver, it is typically required to drive heavy loads (e.g., 50 Ohms) with a reasonably small amount of power consumption, perform linearly, and provide a desired level of gain. Often, the linearity of a power amplifier driver is determined by the linearity of its voltage-to-current converter (i.e., the transconductance (gm) stage). Given a fixed amount of current, a differential pair of amplifiers' linear performance increases by increasing the amount of its Vgs−Vt (=Vgt). One of average skill in the art readily appreciates that increasing channel length of a field effect transistor further increases Vgt. However, this results in lower gain for a given bias current and is also subject to velocity saturation limits.
Many schemes have been traditionally used to linearize a transconductance stage as compared to that obtained from a standard differential pair, which is shown in FIG. 1. As shown, the transconductance stage includes a pair of transistors operably coupled to receive a differential input voltage and, based on the current provided by the current source, produces a differential output current. However, the linearization of the transconductance stage shown in FIG. 1 is limited.
FIG. 2 illustrates a transconductance stage that improves linearity, with respect to the transconductance stage of FIG. 1. In this implementation, resistors are added in series with the input transistors. The resistors increase the linear operational range of an amplifier through the local series feedback. This improvement, however, is at the expense of reduced gain, reduced headroom, and increased noise. One solution that has been recognized heretofore is to compensate for the reduction in gain by adding additional transconductance stages. This approach, however, consumes more current, integrated circuit real estate and consumes more power.
FIG. 3 illustrates an alternate transconductance stage that includes inductors in series with the input transistors. This transconductance stage is an improvement over the transconductance stage of FIG. 2 in that it requires less operating voltage and does not contribute extra noise to the output current. However, it still has an effective reduction of the gain and works over a narrow frequency range.
FIG. 4 illustrates yet another known implementation of a transconductance stage. In this instance, the input transistors are operably coupled to an effective ground wherein the inputs are AC coupled and biased to a particular bias voltage. The implementation of FIG. 4 results in a fundamentally different large signal transfer function in contrast to the large signal transfer functions for the circuitry of the differential pair amplifiers illustrated in FIGS. 1 through 3. This transfer function is typically more linear in nature and requires less headroom than that of a standard differential pair and further has no degeneration noise penalties. However, this circuit in FIG. 4 provides a limited amount of improvement in linearity performance as compared to that of the differential pairs of FIGS. 1 through 3. Such limited linearity in many systems is unacceptable for many applications.
Therefore, a need exists for a DC coupled transconductance stage that operates from low supply voltages, has good noise performance, and has good linearity performance.